Materials having heat resistance of 150° C. or higher are demanded so as to cover (coat) semiconductor elements in semiconductor devices of high heat resistance and high breakdown voltage. Among them, materials (encapsulants) for covering optical materials such as optical semiconductor elements require excelling not only in heat resistance, but also in other properties such as transparency and flexibility. Phenylsilicones are now mainly used as encapsulants typically in backlight units of liquid crystal displays.
Patent Literature (PTL) 1 discloses a resin composition for optical element encapsulation, where the resin composition excels in transparency, ultraviolet resistance, and thermal coloration resistance. The resin composition for optical element encapsulation contains, as a resin component, at least one silsesquioxane selected from the group consisting of a liquid cage-structure silsesquioxane containing an aliphatic carbon-carbon unsaturated bond and containing no H—Si bond; and a liquid cage-structure silsesquioxane containing an H—Si bond and containing no aliphatic carbon-carbon unsaturated bond. The resin composition containing such a cage silsesquioxane, however, gives a cured product that is relatively hard, has poor flexibility, and is disadvantageously susceptible to cracking and fracture.
PTL 2 discloses a curable composition that contains a triallyl isocyanurate or another organic compound containing at least two carbon-carbon double bonds per molecule, where the carbon-carbon double bonds are reactive with SiH group; a chain and/or cyclic polyorganosiloxane or another compound containing at least two SiH groups per molecule; and a hydrosilylation catalyst as essential components. These materials, however, are still insufficient in properties such as cracking resistance.